This invention lies in the field of motor control systems for stepper motors and more particularly, motor control systems adapted to drive stepper motors in typewriters and like printing instruments, control being based upon tracked position of the element being driven by the stepper motor.
In the field of electronic typewriters, and in particular the latest generation of intelligent typewriters, printing systems are provided which utilize highly automated control systems for providing high speed word processing capabilities. In such control systems, multiple phase stepper motors have been found to be highly efficient for transporting or rotating elements, i.e. a linear stepper motor for driving the carriage, and a rotary stepper motor for rotating the daisy wheel which is in turn carried on the linearly driven carriage. The construction of the motors is known in the art, and reference is made to an article titled MOTION CONTROL ASPECTS IN THE QYX INTELLIGENT TYPEWRITER, by G. Singh, M. Gerner and H. Itkowitz, presented at the 8th annual symposium on Incremental Motion Control Systems And Devices, Champagne, Ill., May 1979. This article is incorporated by reference and specifically referred to as background for typewriter systems, and in particular for the operation and construction of stepper motors.
In the prior art as described by the Singh et al article, motion monitoring is done for the limited purpose of detecting when the rotor has advanced to the point where the phase energization should be switched. Thus, in this prior art system the selection of the phase pair to be energized is done on a predetermined or programmed basis, and the position information is used only to initiate switching of the phase windings that are energized. In such a prior art system, there is in fact no actual tracking of the position of the rotor or of the element such as the daisy wheel which is being driven thereby, and thus the control system has no reliable way of knowing where in fact it is. Although prior art position counting systems as such are generally known in various fields, such systems suffer the detriment of not being able to detect when or if an error in position calculation has occurred, so as to be able to correct error position data.
There thus has remained a need in the art, and particularly in the typewriter art, for a control system which drives the moving element from a present position to a target position, the control system operating as a function of tracked position, wherein there is provided reliable means of obtaining position data and knowing when or if there is an error in position, so that proper position error recovery can be carried out.